Nanostructured electroluminescent device and display

ABSTRACT

An electroluminescent device contains (1) first and second electrodes, at least one of which is transparent to radiation; (2) a hole conducting layer containing first nanoparticles wherein the hole conducting layer is in contact with said first electrode; (3) an electron conducting layer containing second nanoparticles where the electron conducting layer is in contact with the hole conducting layer and the second electrode; and optionally (4) a voltage source capable of providing positive and negative voltage, where the positive pole of the voltage source is connected to the first electrode and the negative pole is connected to the second electrode. In some embodiments, the electroluminescent device also includes an electron-hole combination layer between the hole and electron conducting layers.

CLAIM OF PRIORITY

This application is a continuation application of U.S. patentapplication Ser. No. 11/676,912, filed Feb. 20, 2007, which claims thebenefit of U.S. Provisional Application Ser. No. 60/774,794, filed Feb.17, 2006, under 35 U.S.C. §119(e) and is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The invention relates to electroluminescent devises and emissivedisplays containing them.

BACKGROUND OF THE INVENTION

Emissive displays fall under three categories depending on the type ofemissive device in the display: (1) Organic Light Emitting Displays(OLED), (2) Field Emission Displays (FED) and (3) Inorganic Thin FilmElectroluminescent Displays (EL). Of these three categories, OLEDs havereceived the most attention and investment around the world.Approximately 100 companies are developing various aspects of the OLEDtechnology. Commercial OLED products are in the mobile phone and MP3markets. OLED devices can be made from small molecules (pioneered byKodak) or polymers (pioneered by Cambridge Display Technology). OLEDdevices can also be made from phosphorescent materials (pioneered byUniversal Display Technology). More than 90% of the commercial productsuse Kodak's fluorescent small molecule materials. Polymer materials, onthe other hand, offer lower cost manufacturing by using solutionprocessing techniques such as spin coating and ink-jet printing.Polymeric materials are expected to offer a cost effective solution forlarge size (>20″) OLED displays. Phosphorescent materials offer higherefficiencies and reduce power consumption.

OLED displays suffer from several materials based and manufacturingprocess dependant problems. For example, OLEDs have short lifetimes,loss of color balance over time, and a high cost of manufacturing. Thepoor lifetime and color balance issues are due to the chemicalproperties of emissive device in the OLED. For example, it is difficultto improve the lifetime of blue OLEDs because the higher energy in theblue spectrum tends to destabilize the organic molecules used in theOLED. The cost of manufacturing small molecule full color displays isalso very high due the need to use expensive shadow masks to depositred, green and blue materials. Kodak and others have developed whiteOLEDs by using color filter technology to overcome this problem.However, the use of color filters adds cost to the bill of materials andreduces the quality of display. Some of the main advantages of the OLEDdisplay are being taken away by this approach.

Polymeric materials offer a possible route to achieve low cost highvolume manufacturing by using inkjet printing. However, polymers haveeven shorter lifetimes compared to small molecules. Lifetimes mustincrease by an order of magnitude before polymeric materials can becommercially viable.

The next generation emissive display technology is expected to be basedon newly emerging nanomaterials called quantum dots (QD). The emissioncolor in the QDs can be adjusted simply by changing the dimension of thedots. The usefulness of quantum dots in building an emissive display hasalready been demonstrated in QD-OLED. See Seth Coe et al., Nature 420,800 (2002). Emission in these displays is from inorganic materials suchas CdSe which are inherently more stable than OLED materials. Stableblue materials can be achieved simply by reducing the size of thequantum dots.

Display devices made with QDs have quantum efficiencies which are anorder of magnitude lower than OLED. QDs have been combined with OLEDmaterials to improve efficiency. See US2004/0023010. However, thisapproach produces only modest improvement in efficiency while decreasingthe display lifetime and complicating the manufacturing process.

SUMMARY OF THE INVENTION

The electroluminescent device contains (1) first and second electrodes,at least one of which is transparent to radiation; (2) a hole conductinglayer containing first nanoparticles wherein the hole conducting layeris in contact with the first electrode; (3) an electron conducting layercontaining second nanoparticles where the electron conducting layer isin contact with the hole conducting layer and the second electrode; andoptionally (4) a voltage source capable of providing positive andnegative voltage, where the positive pole of the voltage source isconnected to the first electrode and the negative pole is connected tothe second electrode.

In some embodiments, the electroluminescent device also includes anelectron-hole combination layer between the hole and electron conductinglayers. The electron-hole combination layer can be a layer of metal ormetal oxide. It can also be a layer of metal or metal oxide incombination with the first and/or second nanoparticles used in the holeand/or electron conducting layers. The electron-hole combination layercan also be a sintered layer where the aforementioned components aretreated, typically with heat, to coalesce the particles into a solidmass. An electron-hole combination layer can also be made at thejuncture of the hole-conducting and electron-conducting layers by simplysintering these two layers in the absence of metal or metal oxide. Ingeneral, the electron-hole combination layer is 5-10 nanometers thick.

The electroluminescent device can also include a hole injection layerthat is between the first electrode and the hole conducting layer. Thehole injection layer can contain a p-type semiconductor, a metal or ametal oxide. Typical metal oxides include aluminum oxide, zinc oxide ortitanium dioxide, whereas typical metals include aluminum, gold orsilver. The p-type semiconductor can be p-doped Si.

The electroluminescent device can also include an electron injectionlayer that is between the second electrode and the electron conductinglayer. This electron injection layer can be a metal, a fluoride salt oran n-type semiconductor. Examples of fluoride salt include NaF, CaF2, orBaF2.

The nanoparticles used in the hole conducting and electron-conductinglayer are nanocrystals. Exemplary nanocrystals include quantum dots,nanorods, nanobipods, nanotripods, nanomultipods, or nanowires. Suchnanocrystals can be made from CdSe, ZnSe, PbSe, CdTe, InP, PbS, Si orGroup II-VI, II-IV or III-V materials.

In some electroluminescent devices, a nanostructure such as a nanotube,nanorod or nanowire can be included in the hole conducting,electron-conducting and/or electron-hole combination layer. A preferrednanostructure is a carbon nanotube. When nanostructures are used, it ispreferred that the nanoparticles be covalently attached to thenanostructure.

DESCRIPTION OF THE DRAWINGS

FIG. 1 (Prior Art) depicts quantum dots that absorb and emit atdifferent colors because of their size differences. These quantum dotsare of nanometer size. Small dots absorb in the blue end of the spectrumwhile the large size dots absorb in the red end of the spectrum.

FIG. 2 (Prior Art) depicts quantum dots of the same size made from ZnSe,CdSe and PbSe that absorb and emit in UV, visible and IR respectively

FIG. 3 (Prior Art) depicts nanoparticles capped with a solvent such astr-n-octyl phosphine oxide (TOPO)

FIG. 4 depicts nanoparticles functionalized with a linker.

FIG. 5 depicts core-shell nanoparticles functionalized with a linker.

FIG. 6-11 depict various embodiments of the nanostructuredelectroluminescent device.

DETAILED DESCRIPTION

The electroluminescent device contains (1) two electrodes, at least oneof which is transparent to radiation, (2) a hole conducting layercontaining first nanoparticles, and (3) an electron conducting layercomprising second nanoparticles. The first and second nanoparticles aredifferent either in composition and/or size. In addition, the first andsecond nanoparticles are chosen such that the first particles of thehole conducting layer conduct holes while the second particles of theelectron conducting layer conduct electrons. The nanoparticles arechosen so that their relative bandgaps produce a Group II band offset.CdTe and CdSe are nanoparticles that present a Group II band offset.However, different nanoparticles can be chosen having differentcomposition and/or size so long as the conduction and valence bands forma Type II band offset. The electroluminescent device optionally includesa voltage source capable of providing a positive and negative voltage.When present, the positive pole of the voltage source is electricallyconnected to the first electrode and hence to the hole conducting layerwhile the negative pole is connected to the second electrode and henceconnected to the electron conducting layer.

In some embodiments, an electron-hole combination layer is placedbetween the hole and electron conducting layers. The electron-holecombination layer can comprise a metal, a metal oxide, or a mixture of ametal or metal oxide with the nanoparticles of the hole conducting layeror the nanoparticles of the electron conducting layer. In some cases,the metal or metal oxide is in combination with the nanoparticles of thehole conducting layer as well as the nanoparticles of the electronconducting layer. The type of electron-hole conducting layer present inan electroluminescent device will depend upon its method of manufacture.FIG. 6 shows an electroluminescent device without a power source. InFIG. 6, a transparent anode such as indium tin oxide (620) is formed onthe glass substrate (610). A first nanoparticle layer is then depositedfollowed by a second nanoparticle layer. A metal cathode (650) is thenformed on the second nanoparticle layer. The entire device can then beannealed/sintered so as to form a continuous layer between the first andsecond nanoparticle layers and an electron-hole combination layer. Theelectron-hole combination layer is formed between the two layers and ismade from nanoparticles from the hole and electron conducting layers. Itis in this region that electrons and holes combine with each other toemit light when a positive and negative voltage is placed across thedevice. The radiation emitted may be dependent upon the differencebetween the conduction band energy of the electron conductingnanoparticles and the valence band energy of the hole conductingnanoparticles. It is to be understood that the emitted radiation neednot correlate exactly to the differences in such energy levels. Rather,light having energy less than this band gap may be expected.

If a layer of metal or metal oxide is positioned between the first andsecond nanoparticle layers, an electron-hole combination layer isformed. If the metal or metal oxide is placed on the first nanoparticlelayer and then sintered prior to the addition of the second nanoparticlelayer, the electron-hole combination layer comprises not only the metalor metal oxide but also nanoparticles derived from the first layer.Alternatively, the second nanoparticle layer can be deposited upon themetal or metal oxide layer and the device then sintered. In this case,the electron-hole combination layer comprises metal or metal oxide incombination with nanoparticles from the first and second layers. If thedevice is made by first depositing the hole conducting layer, followedby a layer metal or metal oxide and sintered, the electron-holecombination layer comprises metal or metal oxide in combination withnanoparticles from the hole conducting layer.

The electroluminescent device may further contain an electron injectionlayer and/or a hole-injection layer. Referring to FIG. 7, the electroninjection layer is positioned between the second nanoparticle layer andthe cathode. Electron injection layers can include n-typesemiconductors, fluoride salts or metals. The n-type semi-conductor canbe, for example, n-doped silicon while the fluoride salt can be sodiumchloride, calcium chloride or barium fluoride. When fluorides are usedthe layer can be 0.5 to 2 nanometers thick. When metals are used, thelayer can be 5 to 20 nanometers thick.

The hole injection layer (730) can be a p-type semiconductor, a metal ora metal oxide. The metal oxide can be, for example, aluminum oxide, zincoxide, or titanium dioxide whereas the metal can be aluminum, gold orsilver. An example of a p-type semiconductor that can be used as a holeinjection layer is p-doped silicon. In FIG. 8, a hole blocking layer(860) is added to the embodiment previously set forth in FIG. 7.Examples of the hole blocking layers include TiO₂, ZnO and other metaloxides with a bandgap greater than 3 eV.

In addition, an electron blocking layer can be disposed between theanode and the first nanoparticle layer or between the hole injectionlayer and the first nanoparticle layer. Examples of electron blockinglayers include those made from TiO₂.

It is to be understood that an electron injection layer can also act asa hole blocking layer. However, in some embodiments two differentmaterials can be used where one acts as an electron injection layer andthe other a hole blocking layer. For example, an electron injectionlayer can be LiF, BaF or CaF while the hole blocking layer can be TiO₂.

Similarly, at the anode, the hole injection layer can also act as anelectron barrier. However, when different materials are used for thesefunctions, the hole injection layer can be made from Au while theelectron barrier layer can be made from Al₂O₃.

As used herein, the term “nanoparticle” or “luminescent nanoparticle”refers to luminescent materials that generate light upon the combinationof holes and electrons. Luminescent nanoparticles are generallynanocrystals such as quantum dots, nanorods, nanobipods, nanotripods,nanomultipods or nanowires.

Luminescent nanoparticles can be made from compound semiconductors whichinclude Group II-VI, II-IV and III-V materials. Some examples ofluminescent nanoparticles are CdSe, ZnSe, PbSe, InP, PbS, ZnS, CdTe Si,Ge, SiGe, CdTe, CdHgTe, and Group II-VI, II-IV and III-V materials.Luminescent nanoparticles can be core type or core-shell type. In acore-shell nanoparticle, the core and shell are made from differentmaterials. Both core and shell can be made from compound semiconductors.

The nanoparticles of the hole conducting layer have a bandgap such thatholes are easily transferred from the anode to these nanoparticles. Thenanoparticles of the electron conduction layer have a bandgap such thatelectrons can easily transfer from cathode to these nanoparticles.Bandgaps of the materials used for the hole and electron conductinglayers will be complimentary to each other to allow efficientrecombination of holes and electrons in the electron-hole combinationlayer.

Quantum dots are a preferred type of nanoparticle. As in known in theart, quantum dots having the same composition but having differentdiameters absorb and emit radiation at different wave lengths. FIG. 1depicts three quantum dots made of the same composition but havingdifferent diameters. The small quantum dot absorbs and emits in the blueportion of the spectrum; whereas, the medium and large quantum dotsabsorb and emit in the green and red portions of the visible spectrum,respectively. Alternatively, as shown in FIG. 2, the quantum dots can beessentially the same size but made from different materials. Forexample, a UV-absorbing quantum dot can be made from zinc selenide;whereas, visible and IR quantum dots can be made from cadmium selenideand lead selenide, respectively. Nanoparticles having different sizeand/or composition are used in each of the nanoparticle layers.

The luminescent nanoparticle can be modified by reaction with a linkerX_(a)—R_(a)—Y_(b) where X and Y can be reactive moieties such ascarboxylic acid groups, phosphonic acid groups, sulfonic acid groups,amine containing groups etc., a and b are independently 0 or 1 where atleast one of a and b is 1, R is a carbon, nitrogen or oxygen containinggroup such as —CH₂, —NH— or —O—, and n is 0-10 or 0-5. One reactivemoiety (e.g., X) can react with the nanoparticle while the other (Y) canreact with another structure such as (1) the electrode, (2) theelectron-hole combination layer, (3) the hole or electron injectionlayer, (4) the hole or electron blocking layer, or (5) othernanoparticles. In some embodiments, the luminescent nanoparticles areused to decorate nanostructures which are then used in the electronand/or hole conducting layers. The linkers, with or without a secondreactive moiety, can also passivate the nanoparticles and increase theirstability and electroluminescence. They can also improve thenanoparticle solubility or suspension in common organic solvents used tomake the charge conducting layers.

By adjusting the components of X_(a)—R_(n)—Y_(b), the distance betweenthe surface of a nanoparticle and any of the aforementioned structurecan be adjusted to minimize the effect of surface states that canfacilitate electron-hole combination outside of the electron-holecombination layer. The distance between these surfaces is typically 10Angstroms or less preferably 5 Angstroms or less. This distance ismaintained so that electrons or holes can tunnel through this gap fromthe electrodes to the electron-hole combination layer.

As used herein, the term “nanostructure,” “electron conductingnano-structure” or “hole conducting nanostructure” refers to nanotubes,nanorods, nanowires, etc. Electron and hole conducting nanostructuresare crystalline in nature. In general, the nanostructures are made fromwide band gap semiconductor materials where the band gap is, forexample, 3.2 eV for TiO₂. The nanostructures are chosen so that theirband gap is higher than the highest band gap of the photoactivenanoparticle to be used in the solar cell (e.g., >2.0 eV).

Electron conducting nanostructures can be made, for example, fromtitanium dioxide, zinc oxide, tin oxide, indium tin oxide (ITO) andindium zinc oxide. The nanostructures may also be made from otherconducting materials, such as carbon nanotubes, especially single-wallcarbon nanotubes.

Electron conducting nanostructures can be prepared by methods known inthe art. Conducting nanostructures can also be prepared by usingcolloidal growth facilitated by a seed particle deposited on thesubstrate. Conducting nanostructures can also be prepared via a vacuumdeposition process such as chemical vapor deposition (CVD),metal-organic chemical vapor deposition (MOCVD), Epitaxial growthmethods such as molecular beam epitaxy (MEB), etc.

In the case of nanotubes, the outside diameter of the nanotube rangesfrom about 20 nanometers to 100 nanometers, in some cases from 20nanometers to 50 nanometers, and in others from 50 nanometers to 100nanometers. The inside diameter of the nanotube can be from about 10 to80 nanometers, in some cases from 20 to 80 nanometers, and in othersfrom 60 to 80 nanometers. The wall thickness of the nanotube can be10-25 nanometers, 15-25 nanometers, or 20-25 nanometers. The length ofthe nanotube in some cases is 100-800 nanometers, 400-800 nanometers, or200-400 nanometers.

In the case of nanowires, the diameters can be from about 100 nanometersto about 200 nanometers and can be as long as 50-100 microns. Nanorodscan have diameters from about 2-200 nanometers but often are from 5-100or 20-50 nanometers in diameter. Their length can be 20-100 nanometers,but often are between 50-500 or 20-50 nanometers in length.

As described above, the electroluminescent device (without a voltagesource) does not include an organic hole conducting polymer or anorganic electron conducting polymer. Except when organic linkers areused, the device is essentially entirely inorganic.

The electroluminescent devices can be used in emissive displays.Emission displays include flat panel displays (alone or in combinationwith other components associated with a finished product) as well asother electronic devices.

EXAMPLES Example 1

A nanostructured electroluminescent device is shown in FIG. 6, atransparent conducting layer ITO 620 is deposited on glass substrate(610) by following methods well known in the art. The surface of the ITOcan be exposed to plasma treatment or other processes well known in theart to adjust the work function of ITO. A first charge conductingnanoparticle layer (630) is then deposited on the ITO layer. Spincoating or ink jet printing or other printing process can be used todeposit nanoparticles dispersed in a suitable solvent. A continuous pinhole free nanoparticle layer can be obtained by heating the substrate toabout 20° C. for about 15 minutes to drive off the solvent. Thenanoparticles in layer 630 can be dots, rods or wires. The firstnanoparticle layer in this embodiment is made from CdSe. Secondnanoparticle layer (640) is deposited directly on top of the firstnanoparticle layer (630). Spin coating or ink-jet printing or otherprinting process can be used to deposit nanoparticles dispersed in asuitable solvent. A continuous pin hole free nanoparticle layer can beobtained by heating the substrate to about 20° C. for about 15 minutesto drive off the solvent. The nanoparticles in layer 640 can be dots,rods or wires. The second nanoparticle layer (640) in this embodiment ismade form CdTe. The particle size of the CdSe in the first nanoparticlelayer (630) and CdTe in the second nanoparticle layer (640) can beadjusted to obtain the desired emission colors. To produce blue emission3 micron dots can be used. To produce red emission 6 micron dots can beused. Other colors can be produced by adjusting the nanoparticle size byusing methods well known in the art. Interface between the twonanoparticle layers can be improved by heating the substrate in asaturated CdCl₂ solution in methanol or by methods well known in theart. Such a treatment creates a suitable interface between the firstnanoparticle layer and the second nanoparticle layer such that efficientelectron-hole combination occurs at the interface. A aluminum metalelectrode (650) is then deposited on top of the second nanoparticlelayer to complete the nanostructured electroluminescent device.

Example 2

Another embodiment of a nanostructured electroluminescent device isshown in FIG. 7. A transparent conducting layer ITO (720) is depositedon glass substrate (710). As described in Example 1, a hole injectionlayer (730) such as aluminum oxide is deposited on ITO layer 720 by themethods known in the art. The first and second nanoparticle layers (740and 750), are then deposited as described in Example 1. An electroninjecting layer (760) such as LiF is then deposited on top of the secondnanoparticle layer by methods well known in the art. An Aluminum metalelectrode (770) is deposited on top of the second nanoparticle layer tocomplete the nanostructured electroluminescent device.

Example 3

Another embodiment of a nanostructured electroluminescent display shownin FIG. 8. The ITO hole injection and first and second nanoparticlelayers are formed as described in Example 2. A hole blocking layer madeof TiO₂ (860) is deposited on top of the second nanoparticle layer bythe methods well known in the art. An electron injecting layer (870)such as LiF is then deposited by methods well known in the art and anAluminum metal electrode (880) is deposited on top of the secondnanoparticle layer to complete the nanostructured electroluminescentdevice.

Example 4

Another embodiment of a nanostructured electroluminescent display shownin FIG. 9. The ITO layer (920) is deposited on glass substrate (910), asdescribed in Example 1, the first nanoparticle layer (930) is thendeposited on the ITO layer as described in Example 1. The nanoparticles(CdSe dots, rods, bipods, tripods, multipods, wires) in this example areassociated with a nanostructure such as the first nanoparticle layer(930) in this embodiment, made by decorating a functionalized singlewall carbon nano tube (SWCNT). The second nanoparticle layer (940) isdeposited directly on top of the first nanoparticle layer (930). Asdescribed in Example 1, the nanoparticles in the second layer (940),functionalized CdTe dots, rods, bipods, tripods, multipods or wires areassociated with functionalized single wall carbon nanotubes (SWCNTs). Analuminum metal electrode (950) is then deposited on top of the secondnanoparticle layer to complete the nanostructured electroluminescentdevice.

Example 5

Another embodiment of a nanostructured electroluminescent display shownin FIG. 10. The ITO, first and second nanoparticle and metal cathodelayers are formed as described in Example 4. However, in thisembodiment, an electron injecting layer (1060) such as LiF is depositedon top of the second nanoparticle layer before an aluminum metalelectrode (1070) is deposited on top of the second nanoparticle layer.

Example 6

Another embodiment of a nanostructured electroluminescent device shownin FIG. 11. This device is made as described in Example 5, except a holeblocking layer (1160) is deposited on top of the second nanoparticlelayer.

The thickness of the ITO layer used in the above embodiments is 100 nmand the thickness of the aluminum layer is 150 nm. The hole injectionlayer is about 5 Angstroms thick and the thickness of the electroninjection layer is about 10 Angstroms. The nanoparticle layers have athickness in the 10-100 nm range.

The above embodiments are some examples of the applying the presentinvention. It will be obvious to any one skilled in the art that othermaterials and material combinations well known in the art can be used inplace of the material examples used in the above embodiments to build ananostructure electroluminescent display according to the presentinvention. For example, other transparent conducting materials can beused as anode instead of ITO. Other metal oxides can be used as holeinjection materials instead of aluminum oxide. Other metal halides canbe used as electron injecting materials instead of LiF to build ananostructure electroluminescent display according to the presentinvention. Other metals such as Ag, Ca can be used instead of Aluminumas cathode to build a nanostructure electroluminescent display accordingto the present invention. CdSe and CdTe nanoparticles are used asexamples for the first and second nanoparticle layers. Other luminescentnanoparticles with suitable bandgaps can be used instead of CdSe andCdTe to build a nanostructure electroluminescent display according tothe present invention.

The above embodiments show a bottom emitting display. It will be obviousto any one skilled in the art that a top emitting display can be builtaccording to the present invention by using appropriate cathode andanode materials well known in the art.

1. An electroluminescent device comprising first and second electrodes,at least one of which is transparent to radiation; a hole conductinglayer comprising first nanoparticles; an electron conducting layercomprising second nanoparticles; an electron-hole combination layerbetween said hole and electron conducting layers; and a voltage sourcecapable of providing positive and negative voltage, where the positivepole of said voltage source is electrically connected to said firstelectrode and the negative pole is connected to said second electrode;wherein said hole conducting layer is between said first electrode andsaid electron-hole recombination layer; and wherein said electronconducting layer is between said second electrode and said electron-holerecombination layer.
 2. The electroluminescent device of claim 1 whereinsaid electron-hole combination layer comprises a layer of metal or metaloxide.
 3. The electroluminescent device of claim 1 wherein said firstand said second nanoparticles comprise at least one metal and whereinthe metal of said electron-hole combination layer comprises at least oneof the metals of said first or second nanoparticles.
 4. Theelectroluminescent device of claim 1 wherein said electron-holecombination layer is a sintered layer.
 5. The electroluminescent deviceof claim 1 wherein said electron-hole combination layer is 5-10nanometers thick.
 6. The electroluminescent device of claim 1 furthercomprising a hole injection layer between and in contact with said firstelectrode and said hole conducting layer.
 7. The electroluminescentdevice of claim 6 wherein said hole injection layer comprises a p-typesemiconductor, a metal or a metal oxide.
 8. The electroluminescentdevice of claim 7 wherein said metal oxide comprises aluminum oxide,zinc oxide or titanium dioxide.
 9. The electroluminescent device ofclaim 7 wherein said metal comprises aluminum, gold or silver.
 10. Theelectroluminescent device of claim 7 wherein said p-type semiconductoris p-doped Si.
 11. The electroluminescent device of claim 1 furthercomprising an electron injection layer between and in contact with saidsecond electrode and said electron conducting layer.
 12. Theelectroluminescent device of claim 11 wherein said electron injectionlayer comprises a metal, a fluoride salt or an n-type semiconductor 13.The electroluminescent device of claim 12 wherein said fluoride saltcomprises NaF, CaF2, or BaF2.
 14. The electroluminescent device of claim1 wherein said first and second nanoparticles are nanocrystals.
 15. Theelectroluminescent device of claim 14 wherein said nanocrystals areindependently selected from the group consisting of quantum dots,nanorods, nanobipods, nanotripods, nanomultipods, or nanowires.
 16. Theelectroluminescent device of claim 14 wherein said nanocrystals comprisequantum dots.
 17. The electroluminescent device of claim 14 wherein saidnanocrystals comprise CdSe, ZnSe, PbSe, CdTe, InP, PbS, Si or GroupII-VI, II-IV or III-V materials.
 18. The electroluminescent device ofclaim 14 further comprising a nanostructure in said hole conducting,electron conducting or electron-hole combination layer.
 19. Theelectroluminescent device of claim 18 where said nanostructure comprisesa nanotube, nanorod or nanowire.
 20. The electroluminescent device ofclaim 18 wherein said nanostructure comprises a carbon nanotube.
 21. Theelectroluminescent device of claim 18 wherein said nanoparticle iscovalently attached to said nanostructure.
 22. An electronic devicecomprising the electroluminescent device of claim 1.